Add a forward-filter-backward-sample example

[fritzo]

This example should demonstrate how to use funsor.adjoint to perform exact inference on a foward pass, followed by monte carlo on the backward pass. A discrete HMM example would be nice.

~Blocked by #164~

[fritzo]

@eb8680 is this already completed? IIRC you did this as part of our paper submission?

[eb8680]

is this already completed?

I don't have a backward-sampling example yet, I guess I could add that to the seal example. Let's leave this open for now.